The present invention relates to a catalyst for preparing maleic anhydride (MAn) by gas-phase partial oxidation. The invention further relates to a process for preparing maleic anhydride using the catalyst of the invention.
To carry out a heterogeneously catalyzed gas-phase oxidation, a mixture of an oxygen-comprising gas, for example air, and the hydrocarbon to be oxidized is generally passed through a plurality of tubes which are arranged in a reactor and in which a bed of shaped catalyst bodies is present.
Heterogeneous catalysts based on vanadyl pyrophosphate (VO)2P2O7 (known as VPO catalysts) are used in the industrial oxidation of hydrocarbons such as n-butane to maleic anhydride. The rate of this heterogeneously catalyzed oxidation in the gas phase is limited by internal transport influences. Here, it is not the diffusion of reactants to the outer surface of the catalyst but the speed of diffusion into the pore structure of the catalyst which is the rate limiting step for the reaction.
The mass transfer limitation can be reduced by increasing the porosity of the catalyst. For this purpose, pore formers, i.e. organic compounds which burn out during later heating, are added to the catalytically active composition. However, this optimization is subject to limits because excessively porous shaped catalyst bodies no longer have sufficient mechanical stability.
A further possible way of improving these catalysts is optimization of the geometry of the shaped bodies. This determines both the external surface area of the catalyst and the resistance to the gas flowing through the bed and thus the pressure drop which has to be overcome between the reactor inlet and the reactor outlet caused by the catalyst particles. Although the use of relatively small catalyst particles increases the external surface area and thus the activity of the catalyst, the pressure drop increases greatly at the same time. In addition, variation of the geometry of the shaped bodies is subject to limits in that the catalyst obtained should have sufficient mechanical stability and no excessive fracture should occur, e.g. on filling the reaction tubes. Geometries having parts which project a long way, thin struts and the like generally have unsatisfactory mechanical stability.
A catalyst geometry which has a large external surface area and displays a low pressure drop and satisfactory mechanical stability is therefore sought.
U.S. Pat. No. 4,283,307 discloses an oxidation catalyst for the partial oxidation of n-butane to MAn in the form of a pellet having a central hole.
U.S. Pat. No. 5,168,090 describes shaped catalyst bodies for preparing MAn whose external surface has at least one hollow space and whose geometric volume corresponds to from 30 to 67% of the volume of the hollow-space-free geometric shape and which have a ratio of the external geometric surface area to the geometric volume of at least 20 cm−1. Specifically, U.S. Pat. No. 5,168,090 discloses cylinders having 3 equidistant grooves in the external surface which run parallel to the axis of the cylinder.
WO 01/68245 discloses a catalyst for preparing maleic anhydride by heterogeneously catalyzed gas-phase oxidation, which catalyst has an essentially hollow cylindrical structure which has a particular ratio of the height to the diameter of the through-hole and a particular ratio of the geometric surface area to the geometric volume.
WO 03/078057 describes a catalyst for preparing maleic anhydride, which catalyst comprises a catalytically active composition comprising vanadium, phosphorus and oxygen and has an essentially hollow cylindrical structure and a geometric density dp which satisfies particular conditions.
WO 2007/051 602 describes shaped catalyst bodies for preparing maleic anhydride, where the geometric base body enveloping the shaped catalyst body is a prism and the shaped catalyst body is provided with three through-holes. The shaped catalyst body should have a triangular cross section with rounded corners.
EP-A 1 120 390 describes a process for preparing 1,2-dichloroethane by oxychlorination of ethene over a bed of catalyst particles. The catalyst particles can be present as cylinders having axial holes.
JP 06-170239 discloses shaped catalyst bodies having a cylindrical body and at least two holes for the synthesis of unsaturated aldehydes and carboxylic acids.